Radiation Oncology is a branch which aims at exploiting biological interaction of Radiation within the tissue to promote kill of cancer cells while minimizing damage to surrounding normal tissue with the help of ionizing radiations.
Wilhelm Conrad Roentgen, the great German Physicist and winner of the First Nobel Prize in Physics discovered X-rays in 1895. In early days Low energy X rays delivered as single large dose via cathode ray tubes formed entity of giving radiation for tumors. It was associated with more toxicity due to poor penetration to deeper tumors. Later further advancements lead to use of Cobalt – 60 and Linear Accelerators which could easily penetrate to deeper tissues as well. Presently fractionated radiotherapy, in which a single large dose is divided into multiple small doses, forms the main modality of treatment with much less side effects.
Modalities in Radiation Therapy
Cobalt – 60 – It is a radioactive source having energy of 1.25MeV and half-life of 5.25 years. Each cobalt therapy machine is having one radioactive Co-60 source mounted into its head – also known as Gantry. The disadvantage of Cobalt 60 is that it has a radioactive source inside which needs to be replaced as it continuously decays over a period of time. Apart from that, it is not useful for deep lying tumors. However advantage of cobalt is its price and applicability in developing countries. These machines are now becoming obsolete.
LAs on the other hand have distinct benefits over Cobalt – 60 units. It has usually 2-3 different photons energies available for treatment ranging from 4MV to 20MV. Such a wide variety of photon energies are useful in different types of tumors especially deep seated tumors. Also it produces electrons, which have special application in treatment of superficial tumors especially skin tumors.
Comparison between Cobalt 60 Unit and Linear Accelerator
Radioactive Co-60 [needs to be replaced every 5-6 yrs]
No radioactive source. Instead has a target.
Range of energies [4, 6, 10, 18, 20 MV]
Range of electrons available [4, 6, 9, 12, 15, 18, 20 MeV]
Source decays with time so dose rate decreases
Different dose rates available and those are fixed over time
Increases over time
Same over time
Radiation Leakage through Gantry Head
Less than 0.2%
Able to treat deeper tissue
Cost of machine
Conventional Radiotherapy Planning
The basic aim of radiotherapy is to target tumor and spare the normal tissue surrounding the tumor target. In conventional Radiotherapy Planning the target is marked over the skin surface of the patient using clinical knowledge and sometimes using X-ray based simulator.
The next advancement in the radiotherapy is 3-Dimentional Conformal Radiation Therapy also known as 3DCRT. In this the tumor is delineated using CT scan of the patient. The dose is delivered using beam modifying devices like MLC (Multi Leaf Collimators).
this is the revolutionary technical advancement wherein the intensity of the radiation is modulated in such a way to target only the tumor and beautifully spare the normal critical structures surrounding the tumor especially in critical areas. The Intensity Modulated Radiation Therapy is currently the standard of care treatment in many sites.
You cannot see you cannot treat. So Image Guided Radiation Therapy is the essential tool for the treatment of complex radiation therapy treatments. In IGRT, the tumor is located daily before the radiation beam is turned on, so as to see that radiation is delivered to the intended target only and not the other structures surrounding.
Stereotactic Radio Surgery/Stereotactic Body Radio Therapy – here instead of delivering small doses a very high dose of radiation is delivered to intra or extra cranial target. It is a highly specialised and very much precise technique.
[Brachy means close] Here the radioactive source is kept in close proximity to the tumor so as to deliver very high dose to the tumor. It is used mainly treatment of Cervical Cancers. Currently most of the centres use High Dose Rate Brachytherapy using radioactive source Iridium -192 isotope.
Particles like electrons, proton beam is generated in the particle accelerator and delivered to the target tissue. Its main applications are in paediatric and brain tumors.
It is a special type of Linear Accelerator mounted on a robotic arm which can move in different directions and treat the target. It has specially designed Iris collimators for focussing beam on the target.
It is specially designed machine for the treatment of Brain tumors. It contains many radioactive sources of Cobalt – 60 which focus onto the target in conformal manner.
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